The present invention relates to a method of making finned tubes as components of air-cooled systems or condensers.
U.S. Pat. No. 5,042,574, issued to Cottone et al. on Aug. 27, 1991, discloses a process to connect flattened tubes of aluminum through intervention of an aluminum-silicon solder to serpentine fins of aluminum in a controlled atmosphere furnace. The connection of the serpentine aluminum fins to the flat tubes of steel with the aluminum-silicon solder by means of a brazing process which can be carried out only when the flattened tubes or the fins have been coated with an aluminum based coating beforehand. This additional step is time-consuming and complicates the overall process because various pre-materials must be prepared and the welding zone of the longitudinal weld to close the flat tube about the circumference may not be coated with aluminum. Otherwise, a reliable weld cannot be ensured. The regions of the flat tubes have to be liberated from welding byproducts and then protected against corrosion.
These conventional finned tubes comprised of flat steel tubes with serpentine finned aluminum structures have the further drawback that the brazing operation of the aluminum-plated flat tubes with the finned structures is carried out at comparably high temperatures, exceeding 600° C. In other words, the brazing operation is carried out near the softening temperature of aluminum so that the aluminum fins are prone to denting that adversely affect its functionality. The solder used here is an aluminum-silicon eutectic which melts slightly below the softening point of aluminum. Also, in view of the different thermal expansion coefficients between aluminum and steel, these materials, after undergoing the brazing operation at a temperature of about 600° C., warp after the subsequent cool down period, and may ultimately lead to a disintegration of the brazed areas.
Japanese patent publication JP 590 82 114 A discloses a method of applying a helical aluminum fin onto a steel tube, whereby an aqueous tin-chloride/zinc-chloride solution is used for brazing. Zinc-chloride is, however, not stable in an aqueous solution and dissociates instantaneously to zinc oxide which also precipitates. In addition, zinc chloride is highly hygroscopic. A batch of zinc chloride disintegrates under normal atmospheric pressure in seconds to an aqueous solution or dispersed milk through hygroscopic absorption of water. Furthermore, when operating with solvents that are free of water (explosion protection), the solvents must be covered with an aluminum foil during application for reasons of condensation and moisture protection. When large surfaces are involved, this method would not be viable economically. Tin does not form an eutectic with aluminum during brazing at low level. A chemically intimate bond, in which diffusion processes are relevant, is not realized.
European patent publication EP 0 845 321 A1 discloses a process for brazing a die-cast aluminum material, using cesium-aluminum tetrafluoride as fluxing agent.
It would be desirable and advantageous to provide an improved method of joining waved fin-like structures of aluminum with the broad sides of flat tubes of steel to obviate prior art shortcomings and to reduce labor and energy costs as well as material consumption.